The long-term goal of this study is to characterize fully the development of a nerve-muscle system, in order to use it as a model in which to study mechanisms underlying muscle morphogenesis, neuromuscular specificity, programmed cell death and hormonal regulation of development. The nerve-muscle system that is being studied is the rectus abdominis muscle of the leopard frog, Rana pipiens. This is a segmented muscle composed of a mixture of fiber types. It develops in the ventral abdominal wall of oembryonic frogs and undergoes extensive remodeling during late larval life and metamorphosis. The results thus far obtained suggest that the muscle will serve as an excellent model in which to study all of the phenomena listed above. The current specific aims of the project are to determine 1) the sites of addition of new muscle fibers during early and mid-larval stages, 2) the manner in which new fibers form, 3) whether any of the fibers formed during the first half of larval life survive metamorphosis, 4) the number and location in the spinal cord of motor neurons innervating a specific segment of the muscle before and after metamorphosis, and 5) whether individual axons within the muscle are specific with respect to the types of muscle fibers they innervate. Tritiated thymidine will be used to label nuclei in newly forming fibers (aim 1) and to label larval muscle fibers (aim 3). Injections at critical developmental stages will be followed by autoradiography of muscles fixed shortly after injection or fixed at later developmental stages. To accomplish (aim 4), motor neurons will be labeled by retrograde transport of horseradish peroxidase injected into the middle segment of the muscle. The spatial relationships of new muscle fibers, large (i.e., more mature) muscle fibers and satellite cells will be determined by electron microscopic examination of serial thin sections of selected muscle regions. The examination will be made with a view toward deducing how new fibers form and separate from the larger fibers upon which many initially develop (aim 2). The specificity of synaptic contacts will be studied in the same series of sections (aim 5). Attainment of the specific aims listed above will add significantly to the emerging understanding of the development of this neuromuscular system.